The brain regional activities of tyrosine hydroxylase, dopamine-Beta-hydroxylase and tryptophan hydroxylase have been measured as a function of age in Kyoto-Wistar rats (WK), spontaneously hypertensive rats (SHR) and stroke-prone rats (SHR-SP). Small changes in enzyme activity were noted during the developmental phase of the hypertension. Perhaps the most significant change was an increase in tyrosine hydroxylase and dopamine-Beta-hydroxylase in the hypothalamus during weeks 5-8. On an individual basis there was a highly significant correlation between hypothalamic dopamine-Beta-hydroxylase and the increase in blood pressure during this period. In the periphery both plasma norepinephrine and plasma dopamine-Beta-hydroxylase were elevated at 3 weeks of age suggesting an early involvement of the sympathetic nervous system in the development of hypertension.